1. Field of the Invention
The present invention relates generally to the polymerization of polyolefins and, more specifically, to the use of a coolant system and simulation model to control temperature in a polyethylene loop slurry reactor.
2. Description of the Related Art
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various industrial products.
One benefit of polyolefin construction, as may be deduced from the list of uses above, is that it is generally non-reactive with goods or products with which it is in contact. This allows polyolefin products to be used in residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
In the specific example of polyethylene, various types may include, for example, high density polyethylene (HDPE), low density polyethylene (LDPE), and linear low density polyethylene (LLDPE). Applications for HDPE, for example, include the manufacture of blow-molded and injection-molded goods, such as food and beverage containers, film, and plastic pipe. Other types of polyethylene, such as LDPE and LLDPE, are also suited for similar applications. Within each type of polyethylene, there may be various grades tailored to specific applications. Each grade is typically defined by the specifications of the polyethylene properties, such as density and melt index. The different types or grades of polyethylene may be produced using the same loop slurry reactor. For example, a single loop slurry reactor may be used to produce both HDPE and LLDPE, as well as multiple different grades of both HDPE and LLDPE. The reaction conditions (or “recipe”) are adjusted to polymerize different types and grades of polyethylene.
Polyethylene is generally produced in bulk by petrochemical facilities, which have ready access to ethylene, the dual carbon molecular building block of the much longer polyethylene polymer. Various processes exist by which ethylene may be polymerized to form polyethylene. The polymerization process itself is exothermic, or heat-generating, and is typically performed in closed systems where temperature and pressure can be regulated to maximize production. As with any such closed system where heat is generated, some means must be supplied to remove heat and thus to control the polymerization temperature. For loop slurry reactors, a coolant system is typically used to remove heat.
Variations in feedstocks, utility supplies, and reaction kinetics induce variations in the reactor (polymerization) temperature. These variations should be mitigated by the reactor temperature control scheme and the reactor coolant system. The control scheme and coolant system should also accommodate reactor upsets caused, for example, by undesirable slug feed of reactants or by rapidly changing heat transfer behavior in a fouling reactor. As will be appreciated by those skilled in the art, an intricate control scheme is normally used to direct the coolant system to maintain the reactor temperature at a desired set point. The complexity of the temperature control typically involves a cascade control scheme, or in other words, a primary controller (i.e., that maintains reactor temperature) that directs a slave controller (i.e., that maintains coolant temperature). The slave controller may send an output to adjust the position of one or more valves in the coolant system. The polyethylene industry has spent considerable engineering and operating resources to understand and improve reactor temperature control.
Problems with coolant control valve sizing and with tuning of the primary and slave controllers have caused poor temperature control of the polymerization. These problems cause the coolant system to remove too little heat or too much heat from the reactor. Poor temperature control in the reactor increases the cost to manufacture polyethylene. In particular, poor temperature control in the reactor results in a wider design basis for coolant system equipment and thus increases equipment costs. Furthermore, swings in reactor temperature impact reactor stability and can lead to a reactor foul and/or unplanned shutdown. Additionally, polymerization temperature affects the properties of the polyethylene and thus poor control of reactor temperature cause off-spec production of polyethylene.